In the situation that environment protection is increasingly concerned, and the voice for the chromium-free green product is stronger and stronger. The major steelworks in the world are all active to develop chromium-free coatings to substitute chromate coatings for the film of their products.
To apply chromium-free coating film onto the surface of strip steel is rather difficult because there are not good methods and standards for both preparation of chromium-free coating liquid and detection of chromium-free coating film thickness. Especially for large-sized electric machines, after a number of laminates of strip steel sheet is stacked together, uneven thickness of coating film of a laminate or laminates will result in an uneven thickness of the whole stack of laminates. Therefore, how to accurately detect coating film thickness and how to optimize application process of coating film are both stiff challenges in production of strip steels with chromium-free coating film.
In the case of on-line detection of coating film thickness, the prior arts often use ray inspection methods which inspect characteristic elements. Or the prior arts use X ray fluorescence remitter to perform the on-line or off-line detection. In these detection methods, however, it is necessary for a coating liquid to contain some characteristic element, such as chromium, which is easy to emit X-ray fluorescence when it is excited, and the content of the element must be changeless. Therefore, there are only very few coating films whose thickness can be detected by use of these methods, and each set of detection instrument can detect only one characteristic element and so the thickness of only one type of coating film. For producing various types of different-type coating films, it is necessary to equip with multiple sets of thickness detection instrument, which is inconvenient to use in mass production. Even if a type of coating film is kept unchanged, the relative difference of compositions of different batches of coating films still has a non-negligible effect on detection result. Chromium-free coating film does not contain characteristic element Cr, so those methods are helpless for on-line detecting.
Nowadays, thickness of chromium-free coating film is often detected by means of off-line film thickness detection instruments. The detection instruments largely apply the eddy current principle to carry on detection. It takes specimens from the head end and the tail end of a steel strip and measures thickness at multiple points on the work side, middle side, and the driven side, and then takes an average of all the measurements as the thickness of the whole coating film on the steel strip. The advantages of those methods lie in that it is convenient to carry out measuring at many points; however, in those methods, it is necessary to have a typical sample. In addition, because the off-line detection systems themselves and the film thickness reference card both have rather big errors and it is impossible to implement on-line monitor, those methods are unsuitable to manufacture steel strips through a continuous annealing furnace and are prone to bring about an uneven film.